1. Field of the Invention
This invention relates to an improved method of applying white paint by rolls to the outer surface of a cylindrical can body.
2. Description of the Prior Art
Some cylindrical can bodies are formed of soft metals such as aluminum or plated or unplated metals such as steel by various working methods such as deep drawing, multiple drawing and drawing/ironing. The cans are generally coated with protective paints on their outer surface alone or both inner and outer surfaces after they have been formed cylindrically. White paint containing titanium white is commonly used for application to the outer surface of a can body since it also serves as a primer for subsequent printing.
One conventional method of applying white paint to the outer surface of a can body is described by reference to FIG. 1, wherein A is a can body, 1 is a guide trough for feeding can bodies, 2 is a vertical circular passageway along which the can bodies are transported, 3 is an application zone provided midway said passageway, and 4 is a rotary application roll so positioned as to make contact with the outer surface of the can body at said midway application zone. Represented by 5 and 6 are a pair of fountain rolls which contact each other, the roll 5 also contacting the application roll 4 at a contact place 11. Hereunder, the rolls 4, 5 and 6 are referred to generically as an application unit. A scraper roll 7 is beneath and contacts the application roll 4 to wipe the remaining paint from said roll 4.
In operation, the cylindrical can body A is guided by the trough 1 to enter the circular passageway 2 along which it is transported, as supported by a mandrel extending in a direction perpendicular to said passageway, and it reaches the application zone 3. As the can body is passed through the zone 3, all of its outer surface is contacted by the application roll 4. The white paint 9 stored between the fountain rolls 5 and 6 is carried on the outer surface 10 of the rotating roll 5 up to a contact place 11 of the rolls 4 and 5 where part of it is transferred onto the outer surface 8 of the application roll 4 as shown in FIG. 2. A part of the transferred paint is further transferred onto the outer surface of the can body in the application zone 3 as shown in FIG. 2. Usually, a given amount of paint is transferred from the application roll 4 to the can body A while the can makes a little-over the predetermined number of revolutions in rolling-contact relationship with the roll 4.
As a result of that, the greater part of the outer surface of the can body is subjected to the predetermined number of applications of paint whereas the remaining little part is subjected to applications of paint in the number which is more than the predetermined number by one. Hereinafter the remaining little part coated in such a manner is referred to as a multi-coated part. This multi-coated part looks whiter than said greter-part, causing a distinct difference in degree of color lightness.
If more cans are fed per unit time, each can passes the application zone 3 in a shorter period of time during which it makes a predetermined number of revolutions to be applied a given amount of paint, and this requires a corresponding increase in the peripheral speed of the can body. Since the application roll 4 makes rolling contact with the can body, the increase in the peripheral speed of the can must be accompanied by an increase in the rotational speed of the roll 4. And then, an increased amount of paint will be thrown off from the rolls at the place 11 where the transfer of paint occurs as shown in FIG. 2, to form a deposit on the applicator assembly to spoil its normal operation and/or on the supporting mandrel to stain the inner surface of the can that the mandrel supports.